The long term goal of this research is to increase our understanding of how protein phosphatases may be involved in the light-mediated processes in photoreceptors and also the modulation of these processes by centrifugal or efferent input to the visual system. Changes in the level of phosphorylation of specific proteins is one of the major mechanisms by which extracellular signals, including light, influence the function of target cells. Therefore, either the protein phosphatases or the kinases, which are responsible for these changes, or both enzymes may be targets for regulation by a particular stimulus. However unlike the kinases, there are very few studies that have examined what protein phosphatases are present in photoreceptors, how they are involved in light-mediated responses, and how their activity can be modulated by light or other agents. The lamellas ventral eye photoreceptor cell has been used extensively in electrophysiological studies of the photoresponse, as well as the modulation of this response by centrifugal (efferent) fibers from the brain. Recent evidence suggests that, in these cells, light mediates the activities of a specific kinase (calcium/calmodulin-dependent protein kinase) and also a specific, but unknown protein phosphatase (46A PrP), and both enzymes consequently may be involved in the calcium-mediated effects on visual sensitivity. The first phase of the proposed research will be a biochemical and immunological examination of what protein phosphatases are present and their subcellular distribution in these photoreceptors. The second phase will be the identification and isolation of 46A PrP. This study will then focus on the use of biochemical methods to determine (1) whether light stimulates an increase in 46A PrP activity, (2) if calcium is involved in light-stimulated activation of 46A PrP, and (3) if 46A PrP activity is increased in response to efferent activity. This will be the first comprehensive examination of what protein phosphatases are present in photoreceptors. This should serve as a foundation to begin to understand the role of protein phosphatases in various aspects of photoreceptor function, and result ultimately in determining if changes in the expression or activities of specific phosphatases are involved in photoreceptor disfunction.